Processing documents with different values of the domains attribute

When DITA elements are copied from one document to another, processors
need to determine the validity of the copied elements. This copying might occur as the result of a
content reference (conref) or key reference (keyref), or it might occur in the context of an
author editing a DITA document.

A processor can examine the value of the domains attribute and compare the set
of modules listed to the set of modules for which it provides direct support. It then can take
appropriate action if it does not provide support for a given module, for example, issuing a
warning before applying fallback processing.

Documents might have incompatible constraints applied; see Weak and strong constraints for more information about constraint compatibility
checking.

When copying content from one DITA document to another, processors SHOULD determine if the data being copied (the copy source) requires
modules that are not required by the document into which the data is to be copied (the copy
target). Such a copy operation is always safe if the copy source requires a subset of the modules
that are required by the copy target. Such a copy is unsafe if the copy source requires modules
that are not required by the copy target.

When a copy operation is unsafe, processors MAY compare the copy source to the copy target to determine if the copy source satisfies
the constraints of the copy target. If the copy source meets the copy target constraints, the
copy operation can proceed. Processors SHOULD issue a warning that the copy was allowed but the constraints are not compatible.
If the copy source does not meet the constraints of the copy target, processors MAY apply generalization until the generalized
result either satisfies the copy target constraints or no further generalization can be
performed. If the copy operation can be performed following generalization, the processor SHOULD issue a warning that the constraints are
not compatible and generalization had to be performed in order to complete the copy
operation.